1. Field of the Invention
The present invention relates to an electrophotographic photoconductor for use in a copying machine, laser printer and laser facsimile apparatus.
2. Discussion of Background
The Carlson process and other processes obtained by modifying the Carlson process are conventionally known as the electrophotographic methods, and widely utilized in the copying machine and printer. In a photoconductor for use with the electrophotographic method, an organic photoconductive material is now widely used because such an organic photoconductor can be manufactured at low cost by mass production, and causes no environmental pollution.
Many kinds of organic photoconductors are conventionally proposed, for example, a photoconductor employing a photoconductive resin such as polyvinylcarbazole (PVK); a photoconductor comprising a charge transport complex of polyvinylcarbazole (PVK) and 2,4,7-trinitrofluorenone (TNF); a photoconductor of a pigment dispersed type in which a phthalocyanine pigment is dispersed in a binder resin; and a function-separating photoconductor comprising a charge generation material and a charge transport material. In particular, the function-separating photoconductor has now attracted considerable attention.
When the function-separating photoconductor is charged to a predetermined polarity and exposed to light, the light passes through a transparent charge transport layer, and is absorbed by a charge generation material in a charge generation layer. The charge generation material generates charge carriers by the absorption of light. The charge carriers generated in the charge generation layer are injected into the charge transport layer, and move in the charge transport layer depending on the electric field generated by the charging process. Thus, latent electrostatic images are formed on the surface of the photoconductor by neutralizing the charge thereon. As is known, it is effective that the function-separating electrophotographic photoconductor employ in combination a charge transport material having an absorption intensity mainly in the ultraviolet region, and a charge generation material having an absorption intensity mainly in a range from the visible region extending to the near infrared region.
In line with the trend toward high-speed copying process and small-size copying machine, there are increasing demands for high sensitivity, quick response performance and high durability of the electrophotographic photoconductor for use with the electrophotographic copying process.
In terms of durability of the photoconductor in the repeated electrophotographic process, the constituting materials and the structure of the photoconductor have been studied not only to prevent the electrical deterioration, that is, the increase of residual potential and the decrease of charging potential, but also to minimize the scraping of the surface top layer of the photoconductor and increase the mechanical strength of the photoconductor.
With respect to high sensitivity and quick response performance of the photoconductor, the generating mechanism of photocarriers in the photoconductor has been analyzed and intensively studied. The generating mechanism of the photocarriers, which varies depending upon the kind of charge generation material, is reported in many references, for example, in P. M. Borsenberger and D. S. Weiss: Organic Photoreceptors for Imaging Systems, Marcel Dekker (1993) Chap. 5,6.
Such mechanism can be roughly divided into two groups. One is the mechanism for a charge generation material to intrinsically generate the photocarriers by itself. This mechanism will be hereinafter referred to as intrinsic mechanism. A phthalocyanine compound is one representative example of the charge generation materials showing the intrinsic mechanism. The other mechanism of generating the photocarrier is extrinsic (which mechanism will be hereinafter referred to as extrinsic mechanism), and this mechanism can be typically seen in an azo pigment. Namely, such an azo pigment cannot generate the photocarriers without the application of any external factor thereto.
The charge generation material generates an exciton (the charge generation material in an excited condition) when absorbs the light. In the case of the intrinsic mechanism, the exciton (excited charge generation material) forms a geminate pair by the mutual reaction between the exciton and the charge generation material not excited. In contrast to this, the geminate pair is formed by the mutual reaction between the exciton and the charge transport material in the extrinsic mechanism. In any case, the geminate pair thus formed is then dissociated into free carriers.
The exciton of an inorganic charge generation material is directly dissociated into free carriers. Unlike the inorganic charge generation material, the organic charge generation material generates the free carriers through at least two steps of the generation of a geminate pair and the dissociation of the geminate pair into free carriers. In order to improve the sensitivity of the organic photoconductor, therefore, the quantum yield of the free carriers may be increased by increasing the quantum efficiency at each of the above-mentioned steps.
To be more specific, the geminate pair is generated by electron transfer reaction between two molecules which are considered to be a minimum unit. The quantum efficiency in the generation of the geminate pair by the electron transfer reaction is determined by the factors such as the mixing degree of two molecules and the energy level thereof. On the other hand, it is reported that the dissociation of the geminate pair into free carriers depends on the applied electric field, but the detailed mechanism of dissociation of the geminate pair into free carriers has not yet been clarified. Namely, any technique that is capable of promoting the process of dissociation of the geminate pair into free carriers has not been found.
In view of the above-mentioned present conditions, to improve the sensitivity of the photoconductor, there remains the subject how to increase the reaction efficiency in the dissociation of the geminate pair into the free carriers.
To obtain the electrophotographic photoconductor with high photosensitivity, the particular charge generation materials are proposed, as disclosed in Japanese Laid-Open Patent Application 5-32905 or the like. Although those conventional charge generation materials are remarkably effective and the photoconductors using such charge generation materials show high sensitivity, deterioration of such performance cannot be avoided in practice after repeated operations for an extended period of time.
On the other hand, many trials have been made to improve the mechanical durability of the photoconductor. Various low-molecular weight compounds have been developed to obtain the charge transport materials. The film-forming properties of such a low-molecular weight compound are very poor, so that the low-molecular weight charge transport material is dispersed and mixed with an inert polymer to prepare a charge transport layer. The charge transport layer thus prepared using the low-molecular weight charge transport material and the inert polymer is generally so soft that the charge transport layer is easily scraped off during the repeated electrophotographic operations by the Carlson process.
In addition, when the charge transport layer comprises the above-mentioned low-molecular weight charge transport material, the charge mobility has its limit therein. This is because the low-molecular weight charge transport material is contained in the charge transport layer in an amount of 50 wt. % at most. The Carlson process cannot be accordingly carried out at high speed, and the size of electrophotographic apparatus cannot be decreased. The charge mobility can be improved by increasing the amount of such a low-molecular weight charge transport material. In such a case, however, the film-forming properties of the charge transport layer deteriorate.
To solve the above-mentioned problems of the low-molecular weight charge transport material, considerable attention has been paid to a high-molecular weight charge transport material. A variety of high-molecular weight charge transport materials are proposed, for example, as disclosed in Japanese Laid-Open Patent Applications Nos. 51-73888, 54-8527, 54-11737, 56-150749, 57-78402, 63-285552, 1-1728, 1-19049 and 3-50555.
When the photoconductor is fabricated by providing a charge transport layer comprising the above-mentioned high-molecular weight charge transport material and a charge generation layer, the photosensitivity is considerably inferior to that of the photoconductor employing the low-molecular weight charge transport material.